1. Field of the Invention
The present invention relates to a stereo type vehicle monitoring apparatus that executes a fail-safe function when an anomaly occurs with a pair of images taken by stereoscopic cameras.
2. Description of the Related Art
In recent years, attention has been focused on a stereo type vehicle monitoring apparatus using on-board cameras. Each camera has a built-in solid-state image sensing device, such as CCD. The monitoring apparatus determines a positional deviation of an object, that is a parallax, from a pair of images taken by left and right cameras (stereoscopic cameras) and calculates a distance to the object (distance information) by using a principle of triangulation. Then, based on the calculated distance, the monitoring apparatus recognizes the running surrounding (e.g., the distance between an outside object and the self vehicle) and alerts the vehicle driver or performs a vehicle control such as shiftdown, as required.
In a practical application of the stereo type vehicle monitoring apparatus, a fail-safe function needs to be provided to guarantee the safe operation of the apparatus. One of failures to be detected in such a vehicle monitoring apparatus is a brightness unbalance in the pair of images (stereoscopic images) taken by stereoscopic cameras. The brightness unbalance is caused by external causes. In order to secure a precision for calculating the distance to the object or recognizing a three-dimensional object in front of the vehicle, by a stereo method based on the principle of triangulation, the brightnesses of the left and right images are set to be balanced. That is, the brightnesses thereof are set so that overall luminance of the pair of images is substantially same. The unbalance in the brightnesses of the pair of images due to external causes will hinder normal monitoring of the outside of the vehicle. The external causes for such anomaly, for example, include an accidental situation where a view scope of one of the stereoscopic cameras is blindfolded. The anomaly also occurs when the optical system (particularly lens) of one of the cameras is smeared or fogged. Further, when a scene in front is taken through the windshield, partially smeared or fogged windshield or irregular reflection due to raindrops may also cause the brightness unbalance. In order to secure a high level of safety of the stereo type vehicle monitoring apparatus, when there is a brightness unbalance between the left and right images, the fail-safe function needs to be activated, so as to temporarily stop the monitor control.
Since the fail-safe function, which is essential in the practical application of the stereo type vehicle monitoring apparatus, has not been established, however, there is a problem for securing a high level of safety of the apparatus.